The invention relates to a process and apparatus for the production of expanded grids in accordance with the preambles of the two independent claims, as well as to an expanded grid produced pursuant to the process.
The manufacture of expanded grids (also called expanded metals) is based conventionally on the plastic (inelastic) deformation of metal strips provided with staggered cuts. A vertically and laterally movable cutter bar is usually employed for the production of expanded grids (compare, for example, U.S. Pat. No. 3,570,086). The cutter bar, by means of the vertical movement, cuts into a sheet-metal web to produce mutually spaced-apart cuts transversely to the longitudinal direction of the web and, during the further course of the cutting motion, simultaneously stretches the transverse strip of the web that has been freed by the cuts to the required dimension; during this process, the expanded metal is not only bent (inelastically) but also stretched. Thereafter, the cutter bar is laterally displaced with a simultaneous advancing of the sheet-metal web in order to form, with a renewed vertical cutting and stretching motion, the subsequent, staggered cuts and to expand the next transverse strip.
The customary manufacturing procedure can be applied solely to thick metal. Thus, a sheet-metal thickness is required which is large in relation to the bridging web width (compare FIG. 12 of U.S. Pat. No. 3,570,086). And this process permits only the production of relatively coarse grid structures with limited accuracy.
The process cannot be applied to thin sheet-metal strips, as necessary, for example, for the production of the lamellar grids of the packings of mass transfer columns wherein the ratio of grid web thickness to grid web width must be very small (EP-B 0 069 241). In this procedure, no stretching is possible, and the grid webs would tear at the nodal points when using the conventional process.
Such lamellar grids thus had to be manufactured heretofore in an expensive fashion individually by pulling apart the outer rims of a sheet-metal strip provided with staggered cuts (EP-B 0 069 241). The accuracy and, above all, regularity of the grid structure, especially important for the effect of the packings of mass transfer columns, could not be achieved herein, or could be attained only incompletely.
U.S. Pat. No. 4,105,724 discloses a process of another kind wherein a synthetic resin sheeting of PVC is provided with staggered cuts, heated in a heating chamber, and pulled out of the heating chamber faster than being fed into the latter, resulting in a cellular structure which is subsequently hardened by cooling. This is not an expanded-grid method but rather a thermoplastic method. An exactly defined and uniform grid form is neither intended nor achievable in this process inasmuch as the grid exiting from the chamber can still be readily deformed during transport prior to solidification.
Methods of some other type wherein the stretching of the grid or of the perforated metal strip takes places transversely to the conveying or manufacturing direction are furthermore known from GB-A 2 120 138, DOS 19 44 273, and U.S. Pat. No. 3,455,135.
Finally, German Patent 926,424 demonstrates how metal strips can be provided with linear slots by means of revolving cutting wheels.